Sewing machines provided with safety covers have been suggested to keep external objects such as user's fingers away from a vertically moving sewing needle during a sewing operation. Also sewing machines have been suggested where a safety operation of some sort is executed upon optical detection of external objects such as user's fingers approaching the sewing needle.
A safety cover described in JP H11-267388 B (patent document 1) includes an arm coupled rotatably to a lower end of a presser bar, a protective plate formed integrally to a front end of the arm, and a lens provided in an open window of the protective plate. The protective plate of the safety cover is switchable between an active position that covers the front side of the needle bar and a retracted position retracted rearward.
A cloth presser unit of a sewing machine described in JP 2002-306878 A (patent document 2) is provided with two pairs of optical sensors composed of two light-receiving portions and two light-emitting portions. The two upwardly-oriented light-receiving portions are disposed on the upper surface of a presser holder that covers the upper side of a cloth presser foot so as to embrace the vertical movement locus (vertical reciprocating area of the sewing needle) of the sewing needle. The two downwardly-oriented light-emitting portions are disposed so as to confront the two light-receiving portions from above. When light projected from the respective light-emitting portions to the light-receiving portions are blocked by a foreign object, a control unit activates the needle swing/needle release pulse motor to drive the needle-bar release mechanism and block the transmission of drive force to the needle bar.
In order to employ the safety cover described in patent document 1 to a sewing machine, the presser foot needs to be removed from the presser bar. Thus, incorporation of the safety cover is cumbersome and time consuming for the user. Also, even if the safety cover is rotated to the retracted position, the safety cover still becomes an impediment upon needle threading, needle replacement, and presser foot replacement, providing poor workability.
According to the cloth presser unit of a sewing machine described in patent document 2, when light projected from the light-emitting portions to the light-receiving portions is blocked by foreign objects (such as user's fingers) in the vertical movement locus, a detection of foreign object is made. The problem with such configuration is that the detection range that allows detection of foreign object is extremely small. In order to increase the detection range, multiple pairs of light sensors are required, which brings adverse effects to cost reduction and compact spacing.